Is the word flock a concrete noun?

No, the noun flock is a concrete noun; a Word for a group of birds or animals; a Word for a physical thing.

An abstract noun is a Word for something that can't be experienced by any of the five physical senses; something that can't be seen, heard, smelled, tasted, or touched. An abstract noun is a Word for something that is known, learned, thought, understood, or felt emotionally.